1. Field of the Invention
The present invention relates to a semiconductor light receiving device, and more particularly, to a semiconductor light receiving device for use in a broadband optical communication system. Advances in the Internet have produced a need, becoming increasingly greater, for a high-speed optical communication system. In optical communication systems, an information transmission rate higher than 40 Gbits/s is required. To meet this requirement, a semiconductor light receiving device capable of operating at a sufficiently high speed is needed.
2. Description of the Related Art
FIG. 10A is a perspective view of a semiconductor light receiving device according to a first conventional technique disclosed in Japanese Unexamined Patent Application Publication No. 2001-127333, and FIG. 10B is a cross-sectional view thereof. A tapered optical waveguide 501 is formed on the surface of a semi-insulating InP substrate 500. A pin photodiode 502, which is buried in an InP region, is coupled with an output end of the tapered optical waveguide 501. The pin photodiode 502 is connected to an n-side electrode 505 and a p-side electrode 506. The thickness of the tapered optical waveguide 501 gradually increases in a direction from its input end toward its output end.
An optical signal inputted into the tapered optical waveguide 501 through its input end travels along the tapered optical waveguide 501 to the photodiode 502. When the photodiode 502 receives the optical signal, the photodiode 502 converts the input optical signal into an electric signal. The resultant electric signal is outputted to the electrodes 505 and 506.
This semiconductor light receiving device using the tapered optical waveguide has as high response performance as capable of operating at 40 GHz. Using this semiconductor light receiving device, an apparatus having high-efficiency performance regardless of polarization has been achieved (N. Yasuda, et at. CPT2001 Technical Digest, (2001), pp. 105).
FIG. 11A is a plan view of a traveling-wave light receiving device according to a second conventional technique disclosed in U.S. Pat. No. 5,270,532 and also in a paper by Kirk S. Giboney published in IEEE Trans. on Microwave Theory and Techniques, Vol. 45, No. 8 (1997), pp. 1310-1319. FIGS. 11B and 11C are cross-sectional views taken along one-dot-chain-line B11—B11 and one-dot-chain line C11—C11, respectively, of FIG. 11A.
A multilayer structure comprising an n-type semiconductor layer 520 located at the bottom, a light receiving layer 511 made of an intrinsic semiconductor located in a middle layer, and a p-type semiconductor layer 521 located at the top is disposed on the surface of a semi-insulating semiconductor substrate 510. This multilayer structure extends along a single straight line. A center electrode 512 is disposed on the surface of the p-type semiconductor layer 521. A ground electrode 513 formed on the surface of the semiconductor substrate 510 is connected to the n-type semiconductor layer 520.
The multilayer structure consisting of the three layers, that is, the n-type semiconductor layer 520, the light receiving layer 511, and the p-type semiconductor layer 521, forms an optical waveguide-type light receiving element. The ground electrode 513 and the center electrode 512 form an electric signal transmission line extending in parallel with the optical waveguide-type light receiving element. An optical signal is inputted to the light receiving layer 511 through its input end. A ground pad 523 is connected to the output end of the ground electrode 513. An output pad 524 is connected to the output end of the center electrode 512.
An optical signal is inputted into an optical waveguide formed by the light receiving layer 511 through its input end, and the optical signal propagates inside the optical waveguide. The propagation of the optical signal causes an electric signal to be generated between the n-type semiconductor layer 520 and the p-type semiconductor layer 521, and the generated electric signal propagates along the electric signal transmission line consisting of the ground electrode 513 and the center electrode 512. A high quantum efficiency can be achieved over a wide band by matching the propagation velocity of the optical signal with the propagation velocity of the electric signal.
FIG. 12A is a perspective view of a velocity-matched traveling-wave light receiving device according to a third conventional technique disclosed in a paper by M. S. Islam et al. published in Microwave Photonics Technical Digest 2000, Oxford, UK, pp. 217, a paper by T. Chau et al. published in IEEE Photonics Technology Letters, Vol. 12, No. 8 (2000), pp. 1055-1057, and in U.S. Pat. No. 5,572,014. FIG. 12B is a side view thereof, and FIG. 12C is a cross-sectional view taken along a plane vertical to a light propagation direction.
An optical waveguide 531 is formed on a semi-insulating semiconductor substrate 530. On the upper surface of the optical waveguide 531, a plurality of photodiodes 532, spaced apart from each other, are disposed along the light propagation direction. Each photodiode 532 is coupled, in an evanescent coupling, with the optical waveguide 531. An electrically conductive film 533 is disposed at one side of the optical waveguide 531 and an electrically conductive film 534 is disposed at the opposite side. The electrically conductive films 533 and 534 form an electric signal transmission line. One electrode of each photodiode 532 is connected to the electrically conductive film 533, and the other electrode is connected to the electrically conductive film 534.
An optical signal propagating through the optical waveguide 531 causes the photodiodes 532 to generate an electric signal. The generated electric signal propagates through the electric signal transmission line consisting of the electrically conductive films 533 and 534. In this light receiving device, the propagation velocity of the optical signal is matched with the propagation velocity of the electric signal so as to achieve high performance to respond a signal at a very high frequency such as several ten GHz.
In the first conventional technique shown in FIGS. 10A and 10B, it is required that the capacitance of the photodiode 502 should be as small as possible to achieve a high speed operation. The capacitance can be reduced by reducing the length of the photodiode 502 in the direction in which light propagates.
FIG. 13A shows the dependence of the capacitance on the length of the photodiode 502. In FIG. 13A, the horizontal axis represents the length of the photodiode 502 in units of μm, and the vertical axis represents the capacitance in units of fF. Herein, the photodiode 502 has a width of 4 μm. As can be seen, the capacitance decreases with decreasing length of the photodiode 502. When the length of the photodiode 502 is 3 μm, the capacitance becomes about 15 fF. When an electric circuit at the following stage connected to the photodiode 502 has an input impedance of 50 Ω, the cutoff frequency determined by a CR time constant becomes as high as 300 GHz, and thus a very high-speed operation is possible.
However, the reduction in length of the photodiode 502 results in a reduction in the absorption of light.
FIG. 13B shows the dependence of the internal quantum efficiency on the length of the photodiode 502. In FIG. 13B, the horizontal axis represents the length of the photodiode 502 in units of μm, and the vertical axis represents the internal quantum efficiency in units of %. As can be seen, the reduction in the length of the photodiode 502 results in a reduction in internal quantum efficiency. Thus, with the first conventional light receiving device, it is difficult to achieve both a high-speed operation and a high efficiency at the same time.
In the second conventional light receiving device shown in FIGS. 11A to 11C, a high-speed operation can be achieved regardless of the capacitance between the n-type semiconductor layer 520 and the p-type semiconductor layer 521. It is desirable that the characteristic impedance of the optical waveguide-type light receiving element be adjusted to 50 Ω so as to match the input impedance of an electric circuit at the following stage connected to this semiconductor light receiving device. In the case where the thickness of the light receiving layer 511 is about 0.2 μm, if the width of the optical waveguide-type light receiving element is adjusted to be about 1 μm, the characteristic impedance of the electric signal transmission line becomes equal to 50 Ω. However, if the width of the optical waveguide-type light receiving element is set to be so small, it becomes difficult to form the center electrode 512 on the upper surface of the optical waveguide-type light receiving element.
In the third conventional light receiving device shown in FIGS. 12A to 12C, not only the characteristic impedance of the electric signal transmission line is adjusted to be 50 Ω, but also other various parameters should be adjusted. Furthermore, in order to match the propagation velocity of the electric signal with the propagation velocity of the optical signal, it is required to adjust the distance of two adjacent photodiode 532 to about 0.15 mm. In the case where 10 photodiodes 532 are disposed to achieve a high quantum efficiency, the length of the light receiving device becomes as great as 1.5 mm or greater.